1. Field of the Invention
This invention relates to methods of holding articles on a substrate and, more particularly, to improved methods of holding an array of oriented semiconductor devices on a substrate during and after the removal of wax that had previously been used to cement the devices to a support.
2. Description of the Prior Art
The invention is particularly useful for the manufacture of small, fragile, devices, such as integrated circuits of the so-called beam-lead type. Examples of these beam-lead devices are disclosed in M. P. Lepselter U.S. Pat. Nos. 3,287,612 and 3,335,338. Although useful with other articles, the invention will be described particularly with respect to the beam-lead type of device which comprises a semiconductor body having integral leads extending from it as cantilever beams. These beam leads provide both the electrical and mechanical connections between the device and an electrical circuit formed on a header or a substrate having a metallized pattern. The leads are formed essentially of gold. A typical beam lead is very small, the leads being only 0.4 of a mil thick, 3 mils wide, and 9 mils long while the semiconductor body may be a square only 2 mils thick and 18 mils on a side.
In the manufacture of these devices, an array of one hundred or more bodies, each oriented in the same direction, is formed in a wafer of semiconductor material, such as silicon. The beam leads are electroplated onto the wafer so that they connect with the bodies to produce complete devices in the wafer. In order to separate the wafer into these individual beam-lead devices, while at the same time maintaining their position and orientation in the array, the wafer is secured to a support with wax with the leads of the devices embedded in the wax. The exposed surface of the wafer is masked by protective squares of photoresist material, each of which covers a device and each of which is spaced from the other. This forms a grid of interstices along which the wafer is etched to separate it into a plurality of individual beam-lead devices. These devices remain cemented to the support with the wax.
In the prior art, in order to free the devices from the support for further handling without disturbing their orientation, the devices were cleaned of the photosensitive coating and temporarily secured to an intermediate holding screen with cellulose nitrate cement. The wax was then dissolved, by means of a solvent which would not attack the cellulose nitrate cement, and the support was removed so that the devices remained on the intermediate holder still oriented but inverted. Next, the exposed portions of the devices were placed against a carrier plate which exerted an attractive force on the device. While held against the carrier plate, the cellulose nitrate cement was removed to release the devices from the intermediate holding screen and leave the devices in their original orientation on the carrier but right side up.
To simplify the processing steps, improved methods were developed. (See W. R. Wanesky U.S. Pat. No. 3,663,326.) However, it is desirable to further simplify these processing steps.
It is essential in the manufacture of the devices that the original orientation of the device be maintained for testing and bonding to electrical circuits. Also, these tiny devices must be transported, without bending the beam leads or otherwise damaging the devices, on a substrate from which they may easily be removed.
As mentioned above, in manufacturing the devices they are cemented with wax to a support. They must be subsequently transferred to a substrate to which they are not cemented. Any cement, which clings to the devices when they are transferred and which would interfere with subsequent manufacturing operations, such as bonding, must be completely removed. It is also desirable to remove this cement quickly.
The devices may be transferred from the support to a silicone resin coated substrate by means of apparatus such as that disclosed in R. I. Strohl and W. R. Wanesky, "Transfer of Integrated Circuits from Hot Wax," technical digest, No. 19, New York, Western Electric Co. (July 1970) pp. 45-46. The devices may also be transferred using more sophisticated apparatus such as that disclosed in J. S. Kerecz, S. Kuba and W. R. Wanesky, "Integrated Circuit Sorting Machine," technical digest, No. 21, New York, Western Electric Co. (January 1971) pp. 21-22. In both cases the apparatus maintains the orientation of the devices.